lottie/model: keep a stat of model for future local allocator.

lottie/model: keep a stat of model for future local allocator.
In a complex lottie resource layer counts can exceed above 100 or in
some cases 1000. As in each frame rendering we visit the whole tree
allocating all of them using a local allocator will speed up the tree
visit as well as creation and destruction.

As the number of layer objects are fixed in a resource we can make use of
a dumb monotonic allocator to allocate all objects in one place.

Change-Id: I6c6c8ea83c8d6382609ea58b6640c033f7fd51f3